Conventionally, various devices have been proposed for removing or reducing the concentration of nitrogen oxides (NOx) in a combustion device, and it is proposed in Japanese patent laid open publication No. 63-502086 corresponding to WO87/02025 to decompose the nitrogen oxides contained in the exhaust gas into nitrogen gas, carbon dioxide and water by spraying a water solution of urea into the exhaust gas at a temperature higher than 2,000.degree. F. According to this process, the concentration of nitrogen oxides can be relatively easily reduced from the exhaust gas of a combustion device such as a boiler simply by spraying a water solution of urea into the exhaust gas.
The applicants of this patent application proposed in Japanese patent laid open publication No. 63-502087 corresponding to WO87/02024 to remove nitrogen oxides from exhaust gas at a temperature level of approximately 1,600.degree. F. by adding oxygen-containing hydrocarbons into the water solution of urea that is to be sprayed into the exhaust gas. Other liquid treatment agents have since been proposed for effectively chemically reducing or de-oxidizing the nitrogen oxides contained in exhaust gas at even lower temperatures.
However, when such agents for reducing nitrogen oxides from exhaust gas are used, it is necessary to conduct the combustion process in the combustion device in a highly controlled condition for the agents to be effective in removing the nitrogen oxides. Therefore, from a practical viewpoint, it has been difficult to achieve the desired result which these agents are capable of producing.
This disadvantage can be overcome by carefully controlling the combustion condition of the combustion device but this approach limits the usefulness of the combustion device or substantially increases the cost for operating the combustion device. Further, there is a need to carefully control the concentration and the amount of the agent depending on the combustion condition. However, not only is such a control difficult to achieve by controlling the operation of the feed pump for the agent but also the system for carrying out such a control requires complex and large hardware components. An alternative approach is to use a plurality of such agents and mix them in different compositions depending on the combustion condition, but this also causes increases the complexity and the size of the overall system.